Electrodeposition of silver from a high silver cyanide content bath



Patented Feb. 3, 1948 ELECTRODEPOSITION OF SILVER FROM A HIGH SILVER CYANIDE CONTENT BATH Joseph S. Williams, New Augusta, and Walter R. Binai, Indianapolis, Ind., assignors to P. R. Mallory & 00., Inc., Indianapolis, Ind., a corporation of Delaware No Drawing. Application May 5, 1942, Serial No. 441,866

9 Claims. (01. 204-46) This invention relates to silver plating.

An object of the invention is to improve silver electrodeposition processes, the plating baths used in such processes and the product of such processes.

Another object is to produce smooth and bright silver deposits of substantial thickness by a process capable of use on a large production scale.

A further object is to produce silver electrodeposits suitable for use as bearing surfaces in internal combustion engines and the like.

Other objects of the invention will be apparent from the following description taken in connection with the appended claims.

The present invention comprises the combination of elements, methods of manufacture, and the product thereof brought out and exemplified in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the combination of elements Without departing from the spirit of the invention.

This invention relates to silver electrodeposits and means for obtaining them and particularly to the thicker types of deposits such as are suitable for bi-metals, inlays, overlays, silver lined bearings, electric contacts, commutators, slip rings and other types of silver deposits. We have found that silver plated steel shells made according to the present invention can be used successfully for bearings for airplane engines. The silver can also be deposited in the form of inlays in copper base backings for use as electric contacts.

It has generally been recognized in the past that the use of higher concentrations of silver cyanide, potassium carbonate and free cyanide and an increase of temperature permits silver plating at higher current densities. Agitation of the plating solution also makes possible increased plating speeds.

Since higher current densities permit more rapid production it may be assumed that electroplaters have heretofore used the highest current densities which they have found to be consistent with good results.

The usual current densities used for silver plating in the past have been in the order of 3 to 7 amperes per square foot, a density of 10 amperes being considered very high. More recently experiments have been made with higher current densities for extremely thin silver coatings but, so far as we are aware, current densities substantially above those mentioned have not heretofore been found practicable for deposits of substantial thickness.

We have discovered that by departing radically from the values of plating bath concentrations and current densities used heretofore and by using values of a much higher order of magnitude, we have been able to produce smooth and bright silver deposits of substantial thickness on a mass production scale. The quality of the silver deposits we have been able to obtain is, in fact, of such a high degree as to permit their use for aircraft engine bearings.

The process of the present invention can be used for applying silver deposits to various bases such as copper, iron and their alloys, for example.

The plating of steel bearing shells will be described below by way of example.

As smooth a finish as possible should be obtained by machining or grinding the surface of the steel shell which is to receive the silver deposit. The article to be plated is degreased and then electrolytically cleaned in an alkaline cleaning solution. After removal it is desirable to thoroughly rinse the article in water.

Before silver plating it is desirable to apply a copper strike in a copper cyanide bath followed by a rinse and a dip in cyanide solution free of copper and silver. A silver strike is then immediately, applied before the surface has a chance to dry.

Silver strike bath Grams per liter Silver cyanide 2.0 to 2.75 Free potassium cyanide to Potassium carbonate 15 to 20 Silver plating bath Grams per liter Silver cyanide 105 to 115 Free potassium cyanide 140 to 150 Potassium carbonate 65 to 70 Potassium hydroxide 2to A carbon disulphide brightener is used in the form of potassium ,Q anide solutionwhich has been allowed to stand in contact wvith carbon disulphide. One or two 0. 0. per gallon are added to the plating bath as required. Other brighteners, such as sodium thiosulphate may be substituted, if desired.

The pH of the plating bathwill normally fall between 12.6 and 13.4. r.

Plating is effected with a.current.fdensity..of 30 amperes per square foot-of cathode :surface if rocker arm agitation of the solution is used.

With circulation of the solution the current can be increased to between 45 and 250- amperes per square foot. Under these conditions the circu1ation should be such as to produce a flow of .30

to 40 linear feet of solution per minute past the surface being plated.

The' bath should preferably be keptlat' a temperature of 118t0'122F. during plating.

While the above are thepreferredi conditions, given by way of example, it is contemplated that some deviation from these conditions can'be made in some instances.

Thusthe silver plating bath may contain:

Silvercyanide in excess of 90 "grams per liter. 7

Free potassium cyanide, from 20% to 150% of the amount of; silver cyanide.

Potassium carbonate, from-a trace up to 80 grams per liter.

Potassium hydroxide, 2-to 5 grams perliterto bring the pI-I between 12.6 and 13.4.

Sodiumcyanide maybe substituted'for the potassium cyanide in equal molecular proportions, although thev potassium saltgis preferred.

It will be noted thatnthe silver content wve .use is much higher than that used by the-:priorart as well as the amount:offreecyanide. .I-Iowever, the potassium carbonate is proportionallyesma'ller inrelation to the otheroingredients.

The temperatures used should be :kept' between a minimum of 105-F.' and a maximum; of 1120.F.,

preferably notinexcess of 122 F. ;with;carbondisulphide brightener; Withsodiumthiosulphate brightener the temperatures as high as 130.F. may be .used. These plating temperatures are well above those heretofore followed by" the: art.

The current densities used may be'betweensi30 and 250 amperes per square foot, the agitation of the solution being increased forthe? higher densities.

.While the present invention, as. to its? objects and advantages, has =:been.describedrherein as carried out in specific embodiments thereof,; it is not desired to be-limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What'is claimed is:

1. The method of making smooth and bright deposits of silverof substantial thickness which comprises electrodepositing .theifsilver from a plating bath containing 90 to 115.2grams: perl iter of silver icyanide, potassium carbonate "in amounts from .a. trace; to isowgrams: per liter,f and potassiumhydroxi'de suflicient to bringflthe pH between-412:6-:and'..13.4:at;a current density: of at" 4 least 30 amperes per square foot of the surface to be plated.

2. The method of making smooth and bright deposits of silver of substantial thickness which.

comprises electrodepositing the silver from a plating bath containing 90 to 115 grams per liter of silver cyanide, free potassium cyanide of to 150% of the amount of silver cyanide, potassium carbonate in amounts from a trace to 80 grams per liter, and potassium hydroxide sufficient to bring the pH between 12.6 and 13.4

, using a current density of at least amperes per square foot of the surface area to be plated.

3. The. method of making smooth and bright ideposits: of silver; of substantial thickness which 7 comprises electrodepositing the silver from' a platingbathcontaining 90to 115 grams per liter of silver-cyanide; free potassium cyanide of 20% to;1 50of the-:-amount of silver cyanide in the bath, potassium carbonate in amounts from a traceto-SO grams per liter, and potassium hydroxide sufl'icient 'to bring the pH between 12.6 r and 13A using a current density of at least 30 amperes per square foot of the surface area to be plated and maintaining the plating bath-.at a temperature between 1059' arid130F.

4.'The method of making smooth and-bright.

deposits of silver of substantial thickness which comprises electrodepositing the silver .from .,.a

plating bath. containing silver cyanide v90.,to.115 grams per liter, freev potassium ,cyanide .between 20% and v150%of the. amount of .silver cyanide in the bath, potassium carbonate from a trace to grams per liter, and potassium hydroxidesufii cientto bring thepI-I between 12.6 and.13.4.using a current density-of. between 30 and 250-amperes per squarefootof the surface to beplated, the bath being agitated .meanwhile. V

5. The method of making. smooth and bright deposits .of silver. of. substantial thickness which comprises electrode depositing the. silver from a plating bath containing ,silvercyanideto.'115' betweenl lO to 150-grams per liter, potassium carbonate between 65 to 770:; grams perv liter; and potassium hydroxidesufficientto bring the pH between 12.6 vand 13.4 using a current density of between 30. to -60 amperes per square footrof the .surface. tobe plated, the bath being agitated meanwhile.

7. The method of making smooth and zbright deposits of silver of substantial; thickness-which comprises .electrodepositing the 1silver froma plating bath containing: silver cyanide between 115- grams per liter; free potassium cyanide between -1403to grams :per liter, potassium carbonate between 65 to 70 grams per'liter, and potassium hydroxide :sufficient i to 1 bring the pH between 12.6 v and 13.4- usinga current density of.

between 30? to60 mperes per-square foot ofthe surf ace to be plated, theiba-th-being maintained Silver cyanide, 90 to 115 grams per liter;

Free potassium cyanide, from to 150% of the amount of silver cyanide;

Potassium carbonate, from a trace to grams per liter;

Potassium hydroxide sufllcient to bring pH between 12.6 and 13.4.

9. A plating bath for high current density electrodeposition of silver, said bath containing- Silver cyanide, to grams per liter;

Free potassium cyanide, to grams per liter; Potassium carbonate, 65 to 70 grams per liter; Potassium hydroxide, 2 to 5 grams per liter.

JOSEPH S. WILLIAMS. WALTER R. BINAI.

7 REFERENCES CITED The following references are of record in the,

file of this patent: 

